1. Field of the Invention
The present invention relates to a recirculating transfer loop for use in color printers and, more particularly, to a recirculating transfer loop for use in color printers which achieves proper registration of color separations by synchronizing motion in the transfer zone.
2. Description of the Related Art
In the electrophotographic type of printing machine, a photoconductive member is charged to a substantially uniform electrostatic potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing developer material into contact therewith. This forms an image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the developer material thereto in image configuration.
In multi-color electrophotographic printing, in addition to forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are additionally recorded thereon. Each single color electrostatic latent image is developed with toner particles of a color complementary thereto. The process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner particles. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy.
Toner images can be transferred to the copy sheet by an electrical field created by a corona generating device which induces transfer to the copy sheet by spraying a corona discharge having a polarity opposite to that of the toner particles on the photoconductive surface. This causes the toner particles to be electrically transferred to the copy sheet. In transferring multiple toner images, each toner image must be in superimposed registration with one another in order to produce a color copy which is not blurred.
The related art has disclosed printing systems which attempt to improve the accuracy of image transfer by assuring superimposed registration between a sheet and successive toner images.
U.S. Pat. No. 4,849,795 to Spehrley, Jr. et al discloses a sheet transport having a sheet gripper which moves in unison with spaced endless belts for advancing a sheet into registration with a developed image. A motor drive roll advances the belts. The motor is controlled by a servo and encoder which matches the gripper position with a position of a photoconductive belt. The gripper is moved at the same velocity as the photoconductive belt during the transfer of developed images. Between image transfers, the gripper is moved at a greater velocity than the photoconductive belt.
U.S. Pat. No. 4,914,482 to Ammenheuser et al discloses a sheet transport system for a color electrophotographic printing machine. A sheet gripper including a gripper bar mounted on a drum grips a sheet and guides it around the drum across a photoconductive drum. The gripper bar rotates relative to a stationary cylinder at substantially the same angular velocity as that of the drum. Registration pins stick out from the gripper bar and mate with holes in non-image areas of photoconductive drum to ensure that toner images are transferred to a copy sheet in superimposed registration with one another. A drive pulley is connected to a direct drive motor and controlled by a phase locked servo motor and an encoder to mate the registration pins with registration holes in the photoconductive drum.
U.S. Pat. No. 4,552,448 to Davidson discloses a sheet transport system for a multi-color printer which advances a sheet into registration with a developed image on a moving member. The sheet gripper bar is detachably coupled to a drum over a portion of the path of movement. A copy sheet is secured to the gripper bar. The gripper bar advances until it encounters a ramp which detaches the gripper bar from the drum. At this point the gripper bar is secured to the ramp and an arm rotates at an angular velocity close to that of the drum for 180.degree. until it engages a slide. The gripper bar continues until it hits a stop. A motor coupled to an arm is energized in a timed sequence such that the tangential velocity of the gripper bar is equal to or slightly less than that of the drum. This may be done by a phase-locked servo and encoder. As the gripper bar approaches the drum, pins are precisely aligned and registered with holes in the drum. Once coupled, the bar advances with the drum through a transfer cycle so that a successive image can be transferred to a copy sheet in superimposed registration. The gripper bar is always in engagement with the photoconductive drum at exactly the same point.
U.S. Pat. No. 4,733,269 to Kasahara et al discloses a control system for a color copier in which a color document is repeatedly scanned by an optical system to sequentially expose a photoconductive drum to a plurality of separated color components. The system uses a paper size setting circuit to set the size of a transfer paper to be used before a copying operation and two servo circuits to determine transfer times in response to a paper size signal outputted by the paper size circuit. A scanning sensor senses the start of the scan and another sensor senses the instantaneous angular position of a transfer drum.
U.S. Pat. No. 4,781,370 to Weisgerber discloses a sheet gripper for a multi-color printer formed of an open portion which encloses a gripper spindle. A compression spring and an adjustment screw are located on the gripper to bias a gripper tip toward engagement with a gripper pad and permit adjustment of the disengaged gripper position with respect to the spindle and gripper pad.
U.S. Pat. No. 4,799,664 to Burger discloses a gripping mechanism which travels on a closed loop around a revolving path. The gripper mechanism has a hook-like gripper and a spring for urging the gripper to a clamped position.
The related art commonly registers the jaws of a gripper bar having a paper leading edge slot therein to appropriate pins on a photoreceptor drum to prevent relative slip during the period of close proximity. This is a poor solution because the initiation and termination of the contact creates disturbances in the motion of the drum, resulting in unwanted artifacts in the latent image being simultaneously written on the photoreceptor drum by the imaging device.